Bandpass trackers for VSB DTV signal receivers are described by C. B. Patel and the inventor in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVERxe2x80x9d. An in-phase synchronous detector synchrodynes digitized VSB DTV signal to baseband for recovering symbol coding. U.S. Pat. No. 5,479,449 describes a narrow bandpass filter being used to convert the digitized VSB DTV signal to a digital narrow-band double-sideband amplitude-modulation (DSB AM) signal. A quadrature-phase synchronous detector synchrodynes the narrow-band DSB AM signal to baseband for recovering an automatic-frequency-and-phase-control (AFPC) signal for a local oscillator that generates local oscillations used in detecting the VSB DTV signal for digitization. U.S. Pat. No. 5,479,449 specifies that the response of the bandpass filter is centered on the carrier wave frequency and is narrow enough in bandwidth that the modulation of the carrier wave is suppressed respective to the carrier wave itself in the response.
U.S. Pat. No. 5,479,449 also describes alternative bandpass trackers for VSB DTV signal receivers which do not use a narrow bandpass filter to convert the digitized VSB DTV signal to a digital narrow-band DSB AM signal, but instead extract AFPC signal for the local oscillator from the response of a narrowband lowpass filter to the quadrature-phase component of the result of complex synchrodyning of the VSB DTV signal to baseband. This alternative type of bandpass tracker has been favored because the narrowband filtering of AFPC signal can be carried out in the analog regime, without employing a digital filter of considerable complexity.
There is a problem with bandpass trackers for VSB DTV signals that arises because the transmitted VSB DTV channel response rolls off in a band of frequencies flanking the carrier, but the transmitted VSB DTV signal does not use a modulation form free of components in a narrow band of frequencies flanking the carrier. This results in asymmetry of the in-phase sidebands near the carrier, so quadrature-phase synchronous detection will not be completely non-responsive to these sidebands. Consequently, very low frequency components of modulation will at times appear in the AFPC signal for the local oscillator that generates local oscillations used in detecting the VSB amplitude modulation to recover baseband symbol coding. The resulting modulation of the frequency and phase of the local oscillator manifests itself as undesirable intersymbol interference in the recovered baseband symbol coding.
U.S. patent application Ser. No. 09/431,272 filed Nov. 1, 1999 and claiming priority from U.S. provisional application Ser. No. 60/111,016 filed Dec. 4, 1998 describes digital filtering of the digitized final I-F signal supplied to the quadrature-phase synchrodyning circuitry for generating AFPC for the local oscillator in the bandpass tracker. This digital filtering reduces the asymmetry of the in-phase sidebands near the carrier to reduce phase jitter in the AFPC loop arising from VSB modulation of the DTV signal. To assure enough vestigial sideband energy to be able to obtain more symmetrical amplitude-modulation sidebands in the region near carrier, and to avoid group delay in this region being excessively non-uniform, the I-F filtering that precedes digitization of the final I-F signal is designed not to roll-off response in the carrier region very much.
Filtering procedures that avoid rolling off the I-F frequency spectrum in the carrier region also affect the response of the in-phase synchrodyning circuitry to the digitized I-F signal, tending to boost the amplitude of components of the recovered baseband symbol coding occurring at low frequencies near zero frequency. This boost is with respect to the amplitude of higher frequency components of the recovered baseband symbol coding. If this tendency is not corrected by other means, the adaptive channel equalizer that is customarily used with the bandpass tracker will act to compensate for this boost.
The adaptation of the adaptive channel equalizer used with a bandpass tracker is speeded in DTV receivers that embody the invention. In such a DTV receiver a digital filter supplies the in-phase synchrodyning circuitry a shaped spectral response to the digitized I-F signal. The amplitude-versus-frequency response of the digital filter rolls off through the carrier region, so that there is substantially no boost of the amplitudes of lower frequency components of the baseband symbol coding recovered by the in-phase synchrodyning circuitry, with respect to the amplitudes of higher frequency components of the baseband symbol coding such circuitry recovers. When the DTV receiver is initially tuned to a channel, the adaptation of the adaptive channel equalizer proceeds without having initially to compensate for the low-end boost.
The invention is embodied in a radio signal receiver for vestigial sideband digital television signals, which receiver is of the following sort. The radio signal receiver includes a tuner for converting the vestigial-sideband amplitude modulation of a selected digital radio signal to an analog intermediate-frequency signal. The radio signal receiver includes analog-to-digital conversion circuitry for converting the analog intermediate-frequency signal to a digitized intermediate-frequency signal, a digital filter for generating a shaped spectral response to the digitized intermediate-frequency signal, and in-phase synchrodyning circuitry for demodulating the digitized intermediate-frequency signal to recover baseband symbol coding. The radio signal receiver includes adaptive baseband equalization and ghost-cancellation filtering responsive to the baseband symbol coding recovered by the in-phase synchrodyning circuitry. In accordance with the invention, the shaped spectral response of the digital filter is designed to improve the flatness of amplitude response and uniformity of group delay of components of the baseband symbol coding that are close to zero frequency. This relieves the adaptive baseband equalization and ghost-cancellation filtering from having to provide such improvements.